Energy modulation of electron bunches using a terahertz-driven dielectric-lined waveguide

In this thesis, the use of a rectangular dielectric-lined waveguide for energy modulation of electron bunches is presented. The choice of waveguide allows for guided THz pulse propagation with phase velocity matched to the electron velocity. The effect of dispersion, in particular group velocity slippage, has been explored and the choice of a narrow bandwidth THz pulse discussed with regards to the increase in interaction length and minimised group velocity dispersion. A coupler was designed for maximising transmission into the accelerating mode of the waveguide. A non-conventional THz source design was required to generate the correct mode. Modelling of the interaction was performed with different methods and tools so as to investigate the required accuracy of simulations. The use of the Time-Domain (TD) and Particle-in-Cell (PIC) solver in CST Microwave Studio (CST-MWS) was compared with purpose-built simulations in Mathematica. It was established that for narrowband THz pulses the interaction as a function of time delay between THz and the bunch is well approximated by a sinusoidal energy modulation. PIC simulations were used to verify the THz bandwidth and centre frequency for which this approximation was valid. A full structure was designed, manufactured and analysed. THz time domain spectroscopy allowed for measurement of the dispersion relation to compare to the model. Dimensional analysis gave the dimensions of the apertures of the structure. The dimensional analysis showed that, due to a manufacturing error, the waveguide dimensions were larger than designed. Experimental work performed using the CLARA beam at Daresbury Laboratory demonstrated energy modulation of a long, chirped electron bunch. This has potential for use as a bunch diagnostic to assess the time-dependent properties. The THz source was of limited energy, showing that only small laser power is required for such a scheme. An energy spread increase of approximately 8 keV was verified, but full bunch acceleration was not observe

Studies of MYRRHA using thorium fuel

The problem of long-lived radioactive waste has yet to find an acceptable solution, with political opposition to geological storage remaining strong. One possible solution which has often been discussed is to use ADSR systems for its incineration. The advantages of thorium as a fuel in this process are obvious, but few detailed numerical studies have been performed. We investigate the potential for using a thorium fuelled subcritical reactor for the incineration of long-lived minor actinide radioactive waste, based on the welldeveloped MYRRHA reactor design, operating in sub-critical mode. We examine the neutron fluxes and spectra in the reactor, particularly in the In-Pile Section (IPS) regions that would be appropriate for such transmutation, comparing the result from thorium fuel rods with those of the standard uranium/plutonium fuel. From this we present the burn-up rates that would be achieved, both initially and in the longer term as 233U is forme

Acceleration of relativistic beams using laser-generated terahertz pulses

Dielectric structures driven by laser-generated terahertz (THz) pulses may hold the key to overcoming the technological limitations of conventional particle accelerators and with recent experimental demonstrations of acceleration, compression and streaking of low-energy (sub-100 keV) electron beams, operation at relativistic beam energies is now essential to realize the full potential of THz-driven structures. We present the first THz-driven linear acceleration of relativistic 35 MeV electron bunches, exploiting the collinear excitation of a dielectric-lined waveguide driven by the longitudinal electric field component of polarization-tailored, narrowband THz pulses. Our results pave the way to unprecedented control over relativistic electron beams, providing bunch compression for ultrafast electron diffraction, energy manipulation for bunch diagnostics, and ultimately delivering high-field gradients for compact THz-driven particle acceleration.Comment: 8 pages, 4 figure

Group Velocity Matching in Dielectric-Lined Waveguides and its Role in Electron-THz Interaction

Terahertz(THz)-driven dielectric-lined waveguides have applications in electron manipulation, particularly acceleration, as the use of dielectric allows for phase velocities below the speed of light. However matching a single frequency to the correct velocity does not maximise electron-THz interaction; waveguide dispersion typically results in an unmatched group velocity and so the pulse envelope of a short THz pulse changes along the length of the structure. This reduces field amplitude and therefore accelerating gradient as the envelope propagates at a different velocity to the electron. Presented here is an analysis of the effect of waveguide dispersion on THz-electron interaction and its influence on structure dimensions and choice of THz pulse generation. This effect on net acceleration is demonstrated via an example of a structure excited by a single-cycle THz pulse, with a comparison of multi-cycle, lower intensity THz pulses on net acceleration

Design of a Dielectric-lined Waveguide for Terahertz-driven Linear Electron Acceleration

A dielectric-lined waveguide has been designed for use as an accelerating structure in terahertz-driven electron acceleration experiments at Daresbury. Experimental verification of acceleration will take place on Versatile Electron Linear Accelerator (VELA). The choice of a rectangular waveguide structure with sidewall dielectric layers enables tuning by varying the spacing between dielectric slabs to account for potential manufacturing errors. Schemes for coupling free-space single cycle THz pulses into the waveguide have been evaluated and optimised through CST simulation. Comparison of simulation with experimental measurements will also be presented

Association between administration of IL-6 antagonists and mortality among patients hospitalized for COVID-19 : a meta-analysis

IMPORTANCE Clinical trials assessing the efficacy of IL-6 antagonists in patients hospitalized for COVID-19 have variously reported benefit, no effect, and harm. OBJECTIVE To estimate the association between administration of IL-6 antagonists compared with usual care or placebo and 28-day all-cause mortality and other outcomes. DATA SOURCES Trials were identified through systematic searches of electronic databases between October 2020 and January 2021. Searches were not restricted by trial status or language. Additional trials were identified through contact with experts. STUDY SELECTION Eligible trials randomly assigned patients hospitalized for COVID-19 to a group in whom IL-6 antagonists were administered and to a group in whom neither IL-6 antagonists nor any other immunomodulators except corticosteroids were administered. Among 72 potentially eligible trials, 27 (37.5%) met study selection criteria. DATA EXTRACTION AND SYNTHESIS In this prospectivemeta-analysis, risk of biaswas assessed using the Cochrane Risk of Bias Assessment Tool. Inconsistency among trial results was assessed using the I-2 statistic. The primary analysis was an inverse variance-weighted fixed-effects meta-analysis of odds ratios (ORs) for 28-day all-cause mortality. MAIN OUTCOMES AND MEASURES The primary outcome measurewas all-cause mortality at 28 days after randomization. There were 9 secondary outcomes including progression to invasive mechanical ventilation or death and risk of secondary infection by 28 days. RESULTS A total of 10 930 patients (median age, 61 years [range of medians, 52-68 years]; 3560 [33%] were women) participating in 27 trials were included. By 28 days, there were 1407 deaths among 6449 patients randomized to IL-6 antagonists and 1158 deaths among 4481 patients randomized to usual care or placebo (summary OR, 0.86 [95% CI, 0.79-0.95]; P =.003 based on a fixed-effects meta-analysis). This corresponds to an absolute mortality risk of 22% for IL-6 antagonists compared with an assumed mortality risk of 25% for usual care or placebo. The corresponding summary ORs were 0.83 (95% CI, 0.74-0.92; P <.001) for tocilizumab and 1.08 (95% CI, 0.86-1.36; P =.52) for sarilumab. The summary ORs for the association with mortality compared with usual care or placebo in those receiving corticosteroids were 0.77 (95% CI, 0.68-0.87) for tocilizumab and 0.92 (95% CI, 0.61-1.38) for sarilumab. The ORs for the association with progression to invasive mechanical ventilation or death, compared with usual care or placebo, were 0.77 (95% CI, 0.70-0.85) for all IL-6 antagonists, 0.74 (95% CI, 0.66-0.82) for tocilizumab, and 1.00 (95% CI, 0.74-1.34) for sarilumab. Secondary infections by 28 days occurred in 21.9% of patients treated with IL-6 antagonists vs 17.6% of patients treated with usual care or placebo (OR accounting for trial sample sizes, 0.99; 95% CI, 0.85-1.16). CONCLUSIONS AND RELEVANCE In this prospectivemeta-analysis of clinical trials of patients hospitalized for COVID-19, administration of IL-6 antagonists, compared with usual care or placebo, was associated with lower 28-day all-cause mortality